1. Field of the Invention
The invention relates to a process of and apparatus for depositing on moving substrates carbon-rich coatings, and more particularly to a process of and apparatus for creating a plasma, the ions of which accelerate toward a substrate and deposit thereon as a carbon-rich coating. Specifically, the invention relates to a process of and apparatus for powering an electrode which causes the creation of a plasma from a carbon-containing gas within an otherwise evacuated chamber containing the electrode. The electrode becomes negatively biased with respect to the plasma, causing positive ions within the plasma to accelerate toward the electrode where they bombard a moving substrate in contact with the electrode, thereby depositing a carbon-rich coating on the substrate.
2. Background Information
Carbon coatings (also called "films") are known to be quite hard, chemically inert, corrosion resistant, and impervious to water vapor and oxygen. Carbon films often are used as mechanical and chemical protective coatings on a wide variety of organic, polymeric, metallic, and semi-conductive substrates.
Carbon films can be deposited in a variety of forms having differing physical and chemical properties, e.g., graphite, diamond, diamond-like carbon (DLC), diamond-like hydrocarbon, or amorphous carbon. Carbon film coatings typically contain two types of carbon-carbon bonds: trigonal graphite bonds (sp.sup.2) and tetrahedral diamond bonds (sp.sup.3). The term "diamond-like carbon" is applied to noncrystalline materials in which tetrahedral bonds predominate. DLC coatings or films have many of the desirable properties of diamond, such as extreme hardness, extremely low electrical conductivity, a low coefficient of friction, and optical transparency over a wide range of wavelengths.
Several techniques for depositing carbon films, using both solid carbon and hydrocarbon sources, have been developed. Among those using a solid carbon source are sputtering, laser evaporation, pulsed discharge rail gun, and cathodic arc deposition. Deposition methods using a hydrocarbon source include ion beam, microwave plasma, RF plasma, and direct current (DC) plasma discharges. In the latter methods, also known as plasma assisted chemical vapor deposition (PCVD), a plasma is generated from a gaseous hydrocarbon source (by microwave, RF, DC, etc.), and the ions thereof are directed toward a biased electrode in intimate contact with the substrate, whereupon a carbon film is built up on the substrate. In general, higher energy impacting species provide coatings having more sp.sup.3 character that are, therefore, more diamond-like.
A number of unit or batch operations for depositing carbon films have been described. Parallel-plate plasma reactors, comprising a grounded electrode and a powered electrode on which a substrate can be located, are available from a variety of commercial sources such as, for example, Plasma-Therm, Inc. (St. Petersburg, Fla.). Depending on the properties of the substrate, the powered electrode can be heated or cooled. Typically, the reactor containing the substrate is evacuated to a relatively low pressure, e.g., approximately 0.13 Pa, after which a hydrocarbon gas is introduced into the reactor. An RF generator delivers power to the powered electrode sufficient to generate a plasma of the hydrocarbon gas. A negative bias voltage on the powered electrode draws positive ions from the plasma toward the electrode for efficient and rapid coating. However, batch vapor deposition devices generally are not suitable for large scale operation or rapid processing of multiple units, as might be required for an industrial operation.
Continuous (i.e., non-interrupted, multiple-unit) carbon-rich film coating processes also have been described. For example, U.S. Pat. Nos. 5,496,595, 5,360,483, 5,203,924 and 5,182,132 (assigned to Matsushita Electric Industrial Co., Ltd.; Osaka, Japan) describe a DC plasma method of coating one or both sides of a magnetic recording tape. A plasma generated from a hydrocarbon gas is impinged on a continuously moving belt or web by means of a plasma-generating tube. Although these coating processes and apparatus are believed to meet the goals and objectives defined therein, application of RF power to a rotatable electrode so as to form a plasma is not described or suggested. Also, the substrate is not described as being in intimate contact with the plasma creating device.
Continuous reel-to-reel jet plasma carbon-rich film coating processes are described in U.S. Pat. Nos. 5,464,667, 5,286,534 and 5,232,791 (assigned to 3M; St. Paul, Minn.). A hollow-cathode DC plasma device generates plasma from a hydrocarbon gas with the ions of the plasma being projected toward a web moving across a RF-biased drum. Although these coating processes and apparatus are believed to meet the goals and objectives defined therein, RF plasma generation is not described or suggested.
Carbon-film deposition from an RF-generated hydrocarbon plasma onto a continuous, moving web is described in Japanese Patent Application (Kokai) 62-83471. A film base is wrapped around a grounded (i.e., non-powered) electrode and high-frequency voltage is applied to a counter electrode spaced from the rotary electrode to generate a plasma.
The above-described methods, while solving some problems, do not address serious deficiencies that prevent economical continuous carbon film deposition in industry. Some of these problems include:
1) In batch processing, the electrical properties of the electrode (the target electrode in RF, the generating electrode in DC) do not remain constant over time. As deposition on a target substrate proceeds, the gradual build-up of carbon coating increases the substrate/electrode capacitance, thus altering the discharge characteristics of the electrode. This leads to a coating process that is inconsistent over time.
2) In batch processes, flaking or delamination of the coating occurs after the coating thickness exceeds some critical minimum value. Flakes are detrimental because they constitute surface blemishes and can be propelled onto other parts of the substrate.
3) In a continuous DC hollow cathode process, a cathode tube that projects hydrocarbon plasma quickly becomes fouled with build-up of carbon-rich material, decreasing cathode efficiency. Over time, substrates coated using a cathode tube become less and less uniform in the downweb direction due to variations in output from the cathode tube.
4) Cathode tubes indiscriminately apply DLC coatings to all surfaces within a vacuum chamber. Not only is this inefficient, it forces frequent cleaning and upkeep of the vacuum chamber.
5) Because of their design, cathode tubes generally cannot coat targets having a width of more than about 30 cm with a coating having acceptable cross-width uniformity. Industrial processes involving substrates with wider widths would need to be accommodated by use of multiple cathodes for each coating run.
6) Cathode tubes have a natural limitation on the input power used for plasma generation. Above some minimum electrical current, arcing of the carbon electrodes occurs. This causes instability during operation. Thus, higher yields and throughput cannot be achieved merely by increasing power to the cathode.
7) Cathode tubes generate a plasma plume that is approximately the same physical shape as that of the exit orifice of the cathode tube. A three-fold variation in cross-web coating weight has been observed from such a hollow-tube cathode (see, e.g., the aforementioned '667 patent).
8) The same limitations as described in numbers 3-7 above exist in other kinds of discharge tubes, e.g., multiple discharge tubes such as those described in the aforementioned '595 patent and inductively coupled discharge tubes such as those described in U.S. Pat. No. 4,645,977.
The deposition of carbon-rich coatings on moving substrates presents a number of challenges that have not been met by methods and devices described heretofore. An efficient, scalable, continuous (e.g., reel-to-reel) method is highly desirable.